1. Field of the Invention
The present invention generally relates to a color cathode ray tube and, more particularly, to a shadow mask assembly for use in the color cathode ray tube comprising a plurality of finely perforated metal sheets welded together and having a predetermined pattern or apertures for the passage of electron beams.
2. Description of the Prior Art
The color cathode ray tube currently available in the market generally comprises, as shown in FIG. 25 of the accompanying drawings, a highly evacuated glass envelope including a funnel section 1 continued at one end to a neck section 14, a faceplate 2 sealed to the opposite end of the funnel section 1 and having a phosphor deposited screen 3, an electron gun assembly 12 housed within the neck section 14 and including red, green and blue electron guns 12A, 12B and 12C, and a finely perforated color selection electrode member or shadow mask assembly 5 disposed inside the evacuated glass envelope in face-to-face relation with the phosphor deposited screen 3. The faceplate 2 includes a generally rectangular screen plate having a peripheral edge portion bent to provide a peripheral flange. The faceplate 2 is sealed to the funnel section 1 through the peripheral flange thereof and also includes a phosphor deposited screen 3 formed on an inner surface of the screen plate so as to confront the interior of the evacuated glass envelope. The peripheral flange of the faceplate 2 has a plurality of pins 4 secured thereto so as to protrude generally radially inwardly of the evacuated envelope 1.
The shadow mask assembly 5 includes a generally rectangular support frame 9, supported by the pins 4 as will be described later, and a similarly shaped, finely perforated shadow mask 7 having a peripheral edge portion bent to protrude in a direction counter to the screen plate thereby to define a skirt area 8. The perforated shadow mask 7 is mounted on the support frame 9 with the skirt area 8 welded to the support frame 9 by means of a plurality of, for example, 16, weld deposits. The perforated shadow mask 7 has a finely perforated area 23 and a non-perforated area 24 intervening between the perforated area 23 and the skirt area 8, said perforated area 23 of the shadow mask 7 having a multiple of apertures 13 defined therein in a predetermined pattern for the passage therethrough of respective electron beams 6A, 6B and 6C from the electron guns 12A, 12B and 12C of the electron gun assembly 12.
The perforated shadow mask assembly 5 also includes a plurality of expansion compensating couplings generally identified by 10 and interposed between the support frame 9 and the pins 4 on the peripheral flange of the faceplate 2 for compensating any possible thermal expansion, which the perforated shadow mask 7 may suffer from during the operation of the color cathode ray tube, thereby to avoid any possible mislanding of the color electron beams on the phosphor deposited screen 3. Each of the expansion compensating couplings 10 comprises a bimetal piece 15 welded to the support frame 9 and a generally elongated leaf spring 11 welded at one end to the bimetal piece 15, the opposite end of said leaf spring 11 being connected to the associated pin 4 so that the perforated shadow mask assembly 7 can be supported in position inside the evacuated envelope with the perforated shadow mask 7 held in face-to-face relation to the phosphor deposited screen 3. As a matter of practice, the phosphor deposited screen 3 is curved so as to protrude axially outwardly of the evacuated envelope and the perforated shadow mask 7 is correspondingly shaped to keep a generally parallel relationship with the phosphor deposited screen 3 when mounted inside the evacuated envelope.
In the prior art color cathode ray tube of the construction described above, the electron beams 6A, 6B and 6C emitted from the electron guns 12A, 12B and 12C pass through the fine apertures 13, defined in the perforated area 23 of the perforated shadow mask 7 and subsequently impinge upon the phosphor deposited screen 3 to excite elemental color phosphor deposits, e.g., red, green and blue phosphor deposits, on the phosphor deposited screen 3.
As is well known to those skilled in the art, the total number of the fine apertures 13 in the perforated shadow mask 7 for the passage of the electron beams occupies about 15% to about 25% of the total surface area of the perforated shadow mask 7 and, therefore, some of the electron beams travelling towards the phosphor deposited screen 3 collide against non-perforated solid portions of the perforated area 23. The consequence is that the perforated shadow mask assembly 5 is undesirably heated.
Experiments with the conventional color cathode ray tubes have shown that the perforated shawdow mask 7 and the support frame 9 tend to be heated as shown by respective curves C1 and C2 in the graph of FIG. 26. More specifically, when the color cathode ray tube is operated by the application of a high voltage of 28 kv with the beam current of 1 mA, the perforated shadow mask 7 exhibited a change in temperature thereof as shown by the curve C1 wherein the perforated shadow mask 7 underwent a considerable change in temperature for about five minutes subsequent to the start of operation of the color cathode ray tube and gained a generally steady temperature after the lapse of about 30 minutes at a value increased by about 40.degree. C. On the other hand, since the heat capacity of the support frame 9 is higher than that of the perforated shadow mask 7, the support frame 9 was slowly heated and gained a steady temperature after the lapse of about one hour subsequent to the start of operation of the color cathode ray tube as shown by the curve C2.
Once the perforated shadow mask assembly 5 is heated in a manner as hereinabove discussed, the perforated shadow mask 7 undergoes a thermal expansion, known as "doming", to deform generally axially outwardly towards the phosphor deposited screen 3 to such an extent as to result in misalignment of the apertures with the elemental color phosphor deposits on the phosphor deposited screen 3. This misalignment causes the displacement of the electron beams impinging upon elemental color phosphor deposits from the ones upon which they were intended to impinge, that is, a mislanding of the electron beams on the phosphor deposited screen 3. This will now be discussed in detail with reference to FIG. 27.
Referring now to FIG. 27, the perforated shadow mask 7 occupying the position shown by the solid line S before the color cathode ray tube is operated is, with the increase of the temperature thereof, thermally deformed or domed in a direction axially outwardly towards the phosphor deposited screen 3 to occupy a position shown by the broken line S1 while the joint thereof with the support frame 9 remains at a fixed point W. As a result of this doming, any one of the fine apertures 13 for the passage of the electron beams, which has been located at the normal point H is displaced to a location H1 and, therefore, one of the electron beams, for example, the electron beam 6A which is emitted from the electron gun 12A and which were to impinge upon a point P on the phosphor deposited screen 3 will impinge upon a displaced point P1, resulting in the mislanding.
This type of mislanding resulting from the thermal deformation or doming of the perforated shadow mask 7 is generally characterized in that the landing points are displaced in a direction towards the center of the phosphor deposited screen 3 as shown by the arrow a, and results in the excitation of the elemental color phosphor deposits radially inwardly neighboring the elemental color phosphor deposits which ought to have been excited. This in turn brings about an incorrect color reproduction which occurs all over the picture being reproduced by the color cathode ray tube.
With the color cathode ray tube with 21-inch screen having the screen plate whose inner surface has a radius of curvature of 1,350 mm, it has been found that, when such color cathode ray tube was operated by the application of a high voltage of 28 kv with beam current of 1 mA, the landing point at which the particular electron beam should impinge upon the phosphor deposited screen 3 was displaced a distance within the range of 0.05 to 0.08 mm, accompanied by the color misalignment.
On the other hand, it has also been found that, when the support frame 9 for the support of the perforated shadow mask 7 is progressively heated approaching the steady temperature, the support frame 9 having occupied the position shown by the solid line F in FIG. 28 before the color cathode ray tube was operated was thermally expanded in a direction generally radially outwardly to occupy a different position shown by the broken line F1. Consequent upon the thermal expansion of the support frame 9, the perforated shadow mask 7 having occupied the position shown by the solid line S in FIG. 28 before the color cathode ray tube was operated was correspondingly deformed to occupy a position shown by the broken line S2 because the joint W thereof with the support frame 9 was displaced in a direction generally radially outwardly to a position shown by the broken line W1. As a result of this displacement, any one of the fine apertures 13 for the passage of the electron beams, which has been located at the normal point H shown in FIG. 28 is displaced to a location H2 and, therefore, one of the electron beams, for example, the electron beam 6A which is emitted from the electron gun 12A and which were to impinge upon a point P on the phosphor deposited screen 3 will impinge upon a displaced point P2, resulting in the mislanding.
This type of mislanding resulting from the radially outward displacement of the support frame 9 upon the thermal expansion thereof is characterized in that the landing points are displaced in a direction radially outwardly of the phosphor deposited screen 3 as shown by the arrow b, and results in the excitation of the elemental color phosphor deposits radially outwardly neighboring the elemental color phosphor deposits which ought to have been excited. In any event, as is the case with the incorrect color reproduction occurring as a result of the doming of the perforated shadow mask 7 described with reference to FIG. 27, this in turn brings about an incorrect color reproduction which occur all over the picture being reproduced by the color cathode ray tube.
In practice, the foregoing phenomena, that is, the incorrect color reproductions resulting respectively from the doming of the perforated shadow mask 7 and from the thermal expansion of the support frame 9, take place generally in combined fashion during the operation of the color cathode ray tube and, therefore, the thermal expansion of both of the perforated shadow mask 7 and the support frame 9 should be compensated for to achieve the correct color reproduction.
It is the expansion compensating couplings 10 that are used for compensating for the thermal expansions. As hereinbefore discussed with reference to FIG. 25, the expansion compensating couplings 10 are interposed between the perforated shadow mask 7 and the support frame 9 for the support thereof. Since the doming of the perforated shadow mask 7 occurs while the joint between the perforated shadow mask 7 and the support frame 9 is fixed at the position W as discussed with reference to FIG. 27, the use of the expansion compensating couplings 10 allows the joint, which occupies the position W before the operation of the color cathode ray tube, to shift to a position W3 shown in FIG. 27 so that the perforated shadow mask 7 which occupies the position S before the operation of the color cathode ray tube can be shifted generally axially towards the phosphor deposited screen 3 to a position shown by the single dotted chain line S3 in FIG. 27. This forward shift of the perforated shadow mask 7 results in a corresponding shift of the position H of the fine aperture 13 to a position H3 which is substantially aligned with the initial position H in terms of the direction of travel of such electron beam 6A towards the correct landing point P on the phosphor deposited screen 3, thereby minimizing the mislanding of the electron beam 6A.
When it comes to the support frame 9, the heating of the support frame 9 occasioned as hereinbefore discussed with reference to FIG. 28 causes the support frame 9 to expand thermally from the position F to a position shown by the broken line F1 in FIG. 28 with the fine aperture 13 consequently displaced from the point H to a different point H2 as shown in FIG. 28. However, the use of the expansion compensating couplings 10 allows the support frame 9, which occupies the position F before the operation of the color cathode ray tube, to shift in a direction close to the phosphor deposited screen 3 to a position shown by the single dotted chain line S3 in FIG. 28, accompanied by a corresponding shift of the fine aperture 13 from the point H to a point shown by the single dotted chain line H3, which point H is substantially aligned with the initial point P in terms of the direction of travel of the electron beam 6A towards the correct landing point P on the phosphor deposited screen 3, thereby minimizing the mislanding of the electron beam 6A.
The use of the bimetal piece 15 as a constituent of each of the expansion compensating couplings 10 is disclosed in, for example, the Japanese Patent Publications Examined No. 43-26152 (published in 1968), No. 44-3547 (published in 1969), No. 47-3506 (published in 1972) and No. 47-40505 (published in 1972).
However, if the color cathode ray tube is operated to reproduce a picture 16 consisting of a dark region A and a highly bright circular region B as shown in FIG. 29(a), a localized area 7a of the perforated shadow mask 7 which corresponds in position to the highly bright circular region B of the picture 16 being reproduced is heated to a temperature higher than that of the dark region A and is therefore deformed thermally to an extent larger than the remaining portion of the perforated shadow mask 7 as shown in FIG. 29(b). The thermal deformation of the localized portion 7a of the perforated shadow mask 7 is known as a localized doming and this localized doming is a cause of a localized color misalignment.
With the expansion compensating couplings 10 used hitherto in the prior art color cathode ray tubes, it has been found difficult to alleviate the localized color misalignment.
In order to eliminate the problem associated with the occurrence of the localized doming, it has been theoretically established that the use of the perforated shadow mask having an increased wall thickness is effective as disclosed in a Japanese paper entitled "Analysis of Local Doming Phenomenon of Shadow Mask Tubes" published in a bulletin of the Japanese society of television.
In general, the perforated shadow mask 7 is largely manufactured by the use of a chemical etching process such as disclosed in, for example, the Japanese Patent Publication Examined No. 51-9264 published in 1976. According to this chemical manufacturing method, a condition is added that the wall thickness t of the perforated shadow mask 7 and the size Sw of each of the fine apertures 13 for the passage therethrough of the electron beams must satisfy the relationship expressed by the following equation: EQU Sw&gt;0.8 .times.t (1)
It has, however, been found impossible to form such fine apertures in the metal sheet of increased thickness in a close density for the perforated shadow mask which satisfy the above described equation.
More specifically, if the elemental color phosphor deposits are formed on the inner surface of the screen plate of the faceplate 2 with minimized pitch between each neighboring elemental color phosphor deposits in order to increase the resolution of the color cathode ray tube, the fine apertures 13 for the passage of the electron beams therethrough must be correspondingly minimized in size because the perforated shadow mask 7 is responsible for the color selection.
On the other hand, in order to minimize the occurrence of the color misalignment resulting from the thermal deformation of the perforated shadow mask 7 thereby to maintain a desirable color purity, the use of the perforated shadow mask 7 having an increased wall thickness is desirable as discussed in the above mentioned bulletin.
Both of the requirement of minimizing the size of each fine aperture in the perforated shadow mask and the requirement of use of the perforated shadow mask of increased wall thickness are apparently contradictory to the relationship expressed by the foregoing equation (1). The formation of the fine apertures 13 in a single metal sheet for the perforated shadow mask 7 in order to satisfy the both is extremely difficult according to the conventional manufacturing practice as hereinbefore described.
In view of the foregoing, the use of a laminated shadow mask, i.e., the perforated shadow mask comprising a plurality of thin metal sheets welded together at their peripheral edge portions and each having a predetermined pattern of fine holes, is suggested in, for example, the Japanese Laid-open Patent Publication No. 57-138746 published in 1982. According to this publication, the stack of the thin metal sheets is said to resemble the perforated shadow mask of increased wall thickness. FIGS. 30 and 31 of the accompanying drawings illustrate a method of making the laminated shadow mask suggested in the above mentioned publication.
Again, according to the Japanese publication No. 57-138746, two perforated metal sheets 21 and 22 of FIG. 31 of different thickness each having a predetermined pattern of minute holes 13a and 13b which eventually form the fine apertures 13 for the passage of the electron beams therethrough are, as shown in FIG. 30, placed over a positioning jig 27 having a plurality of positioning pins 26 which are, when the metal sheets 21 and 22 are so mounted, engaged in respective positioning holes 25 formed in the non-perforated area 24 of each metal sheet 21 and 22 with the fine holes in one metal sheet 21 exactly aligned with those in the other metal sheet 22. While the metal sheets 21 and 22 are so retained on the positioning jig 27, the non-perforated areas 24 of the respective metal sheets 21 and 22 are welded together by the use of either a spot welding technique or a seamless welding technique.
The stack of the metal sheets 21 and 22 with the fine holes 13a in one metal sheet 21 aligned with the corresponding fine holes 13b in the other metal sheet 22 is shown in FIG. 31(a). After the stack is formed on the positioning jig 27 as hereinabove described, the fine holes 13a and 13b in the respective metal sheets 21 and 22 are filled up with polyamide resin 29 as shown in FIG. 31(b) and is then dried by the application of a heated air to allow the resin deposits 29 in the fine holes 13a and 13b to be cured for imparting a sufficient strength.
Thereafter, in a manner similar to the manufacture of the standard perforated shadow mask using the single thin metal sheet, the stack of the perforated metal sheets 21 and 22 are, as shown in FIG. 31(c), curved to a predetermined curvature with the use of a press comprised of male and female molds. During the stack of the perforated metal sheets 21 and 22 being subjected to the press work, the resin deposits 29 filled in the fine holes 13a and 13b serve to avoid any possible relative sliding motion between the perforated metal sheets 21 and 22 thereby to avoid any possible displacement the fine holes 13a in the perforated metal sheet 21 relative to the fine holes 13b in the perforated metal sheet 22.
Finally, as shown in FIG. 31(d), the resin deposits 29 in the fine holes 13a and 13b in the respective perforated metal sheets 21 and 22 are removed by fusion with the use of either a mechanical means or a laser beam.
However, it has been found extremely difficult to completely remove the resin sticked to the perforated metal sheets 21 and 22 even though the removal of the resin deposits 29 is effected subsequent to the press work and even though the resin deposits within the fine holes 13a and 13b have successfully been removed. This is because a solution of polyamide resin which eventually form the resin deposits 29 has penetrated in and has subsequently cured between the perforated metal sheets 21 and 22. Therefore, during the use of the color cathode ray tube, not only do gaseous impurities generated from the resin tend to flow out into the evacuated envelope resulting in a reduction of the lifetime of the color cathode ray tube, but also the resin remains left unremoved tend to scatter within the evacuated envelope under the influence of vibrations induced by one or more loudspeakers used in the television receiver set. It often occurs that the resin remains which have been scattered within the evacuated envelope may eventually deposit on the electron guns 12A, 12B and 12C to such an extent as to result in a sparking occurring in the electron guns 12A to 12C.
It has also been found that, because of the tendency of the stacked metal sheets 21 and 22 to restore to the original shape after the press molding, the exact alignment between the fine holes 13a in the perforated metal sheet 21 and the corresponding fine holes 13b in the other perforated metal sheet 22 tends to be destroyed.